Delignification and bleaching of cellulose pulp with oxygen gas

ABSTRACT

Apparatus for dividing a stream of pulp into a series of layers containing discrete batches and progressively transferring the batches from layer to layer in controlled fashion so that the height of each layer does not exceed a maximum value at which the pulp at the bottom of a layer has a predetermined minimum gaseous content. The pulp is contacted with oxygen gas under pressure while progressively transferring pulp from one layer to the next in a series. The apparatus comprises a cylindrical pressure vessel including axially spaced floors which define chambers between them, the chambers being subdivided axially into compartments, each floor having an aperture and the compartments and floors being relatively movable to allow pulp in the compartments sequentially to be transferred from one chamber to the next as relative movement occurs.

United States Patent 1191 Verreyne et al.

DELIGNIFICATION AND BLEACHING OF CELLULQSE PULP WITH OXYGEN GASInventors: Abraham Jacob Verreyne, Petersfield Springs; Leonard AustinJob,

Johannesburg, both of Republic of South Africa; Paul Rerolle,Nogentsur-Marne Johan Christoffer Fredrik Carl Richter, Alpes Maritimes,both of France.

A ssignee: South African Pulp and Paper Industries Limited and LAirLiquide, Societe Anonyme Pour LEtude et LExploitation Des ProcedesGeorges Claude, and Aktiebolaget Kamyr Filed: I Jan. 27, 1972. Appl.No.: 221,263

Related US. Application Data Division of Ser. No. 840,639, July l9,1969, Pat. No.

Foreign Application Priority Data July ll, 1968 Sweden 9640/68 July 15,1968 Sweden 9689/68 Jan. 22, 1969 Sweden 868/69 Oct.' 14, 1968 SouthAfrica 68/6629 U.S. Cl. 68/5 D, 259/7 Int. Cl D2lc 9/10, BOlf 15/02 [451Jul 3, 1973 [58] Field b: Search 68/5 R, s c, 5 D, 68/5 E, 181 R; 259/7.8

[56] References Cited UNITED STATES PATENTS 3,041,051 6/l962 Reiffen259/8 X 3,298,209 l/l967 Laakso.... 68/l8l R 3,298,900 l/l967 Laakso68/181 R X Primary Examiner-Wayne A. Morse, Jr. AssistantExaminer-Philip R. Coe

[5 7] ABSTRACT Apparatus for dividing a stream of pulp into a series oflayers containing discrete batches and progressively transferring thebatches from layer to layer in controlled fashion so that the height 'ofeach layer does not exceed a maximum value at which the pulp at thebotbers being subdivided axially into compartments, each floor having anaperture and the compartments and floors being relatively movable toallow pulp in the compartments sequentially to be transferred from onechamber to the next as relative movement occurs.

8 Claims, 6 Drawing Figures CROSS RELATED APPLICATION This Applicationis a division of application, Ser. No. 840,639 filed July 10, 1969 andissued as U.S. Pat. No. 3,660,225 on May 2, 1972.

This invention relates to the delignification and bleaching of cellulosepulps with oxygen gas under pressure.

For the purpose of this specification the term oxygen gas includes anygas containing free oxygen, such as air.

It is known that a pulp can be delig'nified and bleached by subjectingit in an alkaline medium to the action of oxygen gas. French patentspecification No. 1,387,853 discloses the treatment of a chemical pulpwith oxygen gas under pressure in an alkaline medium in the presence ofa catalyst or protector acting to preserve the physical and mechanicalstrength properties of the pulp. South African patent specification No.

' 67/3680 discloses a method of delignifying and bleaching an alkalinepulp by pretreating the pulp with an acidic medium and thereaftersubjecting the pulp in an alkaline medium to the action of oxygen gasunder pressure.

Delignification and bleaching agents other than oxygen gas are known andnormally such other agents react with lignins in an aqueous medium.Generally the correct amount of these other agents can be dissolved inthe aqueous phase of the wet pulp. Oxygen gas treatment must, however,be conducted under pressure and even then only small amounts of oxygencan be dissolved in water at such high oxygen partial pressures. As aresult, it is necessary for the oxygen gas to be applied directly to thepulp mass in sufficiently large quantity if a satisfactory reaction isto be obtained.

It has been found that the amount of oxygen required for a satisfactoryreaction can be accommodated by the pulp only if the bulk density of thepulp is sufficiently treatment of pulps with oxygen gas. Normally, suchtowers operate at relatively low consistencies with no free gas occludedin the pulp. Tests have shown that if such towers are operated at higherconsistencies the pulp compresses under its own weight and liquid isinclined to collect at the bottom of such conventional towers so thatthe oxygen content of the pulp towards the bottom of such towers is toolow for satisfactory reaction.

Although methods of repeated exposure of pulp to a gas are known suchmethods generally suffer the disadvantage of complex mechanisms withconsequent excessive wastage of space between stages and difficulty ofsealing against over pressure as well as high equipment costs.

Methods using a series of trays provided with rake arms causing transferalternatively at the periphery and the centre of the trays are known.These suffer the dis-' advantage that energy is used wastefully due torollback of free flowing materials while sequential flow and control ofresidence time is poor as a consequence. The applicants have found byexperiment that this type of apparatus is incapable of handling beds ofpulp deeper than 0.4 metres in a manner capable of achieving goodresults by the oxygen bleaching process. As a result of this heightlimitation either the number of plough systems must be increased or thediameter of the pressure. vessel must be increased. Both of theseactions cause a high cost penalty.

It is accordingly an object of the present invention to provide improvedtreatment of cellulose pulp with oxygen gas under pressure with whichthe above disadvantages are at least minimized.

Our co-pending U.S. Pat. application, Ser. No. 840,639, now U.S. Pat.No. 3,660,225, discloses a continuous method of treating a cellulosepulp with oxygen gas under pressure comprising the steps of continuouslyintroducing pulp at a consistency of 16 percent to 67 percent into theupper region of a vertically elongated pressure vessel; heating thepulp; distributing the heated pulp in well divided form in a series ofseparate layers individually supported within the pressure vessel atdifferent levels along the length of the pressure vessel; progressivelytransferring pulp downwardly from one layer to the next in the seriesafter a residence period in each layer; contacting the pulp in eachlayer with oxygen gas at a partial pressure in excess of 3 barsabsolute; restricting the height of at least certain of the layers to amaximum value at which the compaction of pulp under its own weight atthe bottom of the layer is limited to a maximum value at which the pulpat the botton of the layer has at least a predetermined minimum oxygengas content; and continuously withdrawing treated pulp from the lowerregion of the pressure vessel.

The minimum amount of oxygen required at the bottom of a layer willdepend on the number of layers in the series and the period of time thepulp is retained in each layer and may also depend on the temperatureand pressure.

Preferably each quantity of' pulp to be transferred through subsequentlayers is determined towards the top of the pressure vessel. The pulpmay be fluffed at the top of the vessel before it is distributed inlayers. The pulp is transferred from one layer to the next in the seriesand away from the last layer in the series under the influence ofgravity. The transfer is effected in stepwise fashion.

An advantage of such stepwise transfer of pulp is that the'pulp mayrepeatedly be transferred through an atmosphere of oxygen Moreover thepulp is subjected to a rearrangement which is favorable for an eventreatment thereof. gas, thereby to re-introduces further oxygen. Thepulp in each layer is subdivided into a plurality of discrete batcheswhich are transferred sequentially in stepwise fashion from one layer tothe next in the series and away from the last layer in the seriessubstantially without intermixing with other batches.

I The support for each layer of pulp is removed sequentially fromunderneath successive batches of pulp in the layer to permit thesequential transfer of successive batches of pulp from one layer to thenext under the influence of gravity, the transfer of pulp from one layerto the next being controlled for a batch of pulp transferred from onelayer to the next to occupy free space in said next layer which haspreviously been vacated by a batch of pulp transferred away from saidnext layer.

The pulp in each layer is collected in a plurality of separatecompartments as discrete batches which are transferred sequentially instepwise fashion from one layer to the next in the series and away fromthe last layer in the series substantially without intermixing withother batches, new batches of pulp being collected sequentially insuccessive compartments in the first layer in the series to replacebatches which are transferred from the first to the second layer in theseries.

The applicants have found that for satisfactory reaction between thepulp and the oxygen gas, the following are important,

a. The pulp should be at a consistency which is sufficiently high forthe pulp to contain enough oxygen and so that there is no substantialdrainage of liquid from the pulp;

b. The pulp is in a well divided form, such as in the form of noodles,or preferably in fluffed form since the liquid retention of fluffed pulpis better than that of noodle pulp and the bulk density of the former islower than the latter;

c. The height of any pulp layer is not too great otherwise the pulpcompacts under its own weight with a resultant expulsion of liquidand/or oxygen at the bottom of the layer.

The applicant's tests have indicated the following:

a. That water begins to drain from softwood noodle pulp at a consistencyof percent under zero pressure and at a consistency of 18 percent undera pressure of a 2.40 metre high pulp layer.

b. The water begins to drain from hardwood noodle pulp at a consistencyof 13 percent under zero pressure and at a consistency of 16 percentunder the pressure of 2.4 metre high pulp layer.

c. That fluffed pulp is less inclined to drain water.

d. That for satisfactory reaction between pulp and oxygen gas, the pulpshould contain at least 1 percent oxygen by weight on a dry pulp basisat the bottom of a layer of pulp. This condition is attained withdifferent pulps with the following maximum layer heights under aselected typical reaction condition of 170 p.s.i.g. and 130C and usingpure oxygen:

Softwood noodle pulp at consistency 3.00 Softwood fluffed pulp at 18%consistency :36 Hardwood noodle pulp at 18% consistency :36

Hardwood fluffed pulp at 18% consistency The heights of these layers canbe increased if the sidewalls exert any friction. However, the object isnot to unnecessarily increase the friction.

In the light of the above results, the method of the present inventionis preferably carried out with pulp at a consistency of not less than 18percent in the case of hardwood noodle and fluffed pulps and of softwoodfluffed pulp and with a pulp consistency of not less than 20 percent inthe case of softwood noodle pulp. Fluffed pulp is to be preferred.

Furthermore, the heights of the various layers may be arranged so as notto exceed 3.00 metres and preferably not to exceed 1.20 metres,especially if lower operating pressures are used.

According to the present invention, apparatus suitable for carrying outthe above method comprises a vertical pressure vessel; spaced deckslocated transversely within the pressure vessel and defining a series ofpulp trapping chambers in the vessel; an outlet port from each chamberlocated in the deck thereof and communicating with an adjacent chamberso as to provide a path for pulp to move gravitationally from chamber tochamber, each oulet port having a radial dimension substantially equalto the effective radial dimension of the chamber; dividing wallsprojecting upwardly transverse to the floor of each chamber to dividethe chamber into a plurality of compartments; means for contacting thepulp with oxygen gas during passage along the pathway; said dividingwalls being stationary; and means for moving the decks of the chambersrelative to the dividing walls to permit the deck of each chamber tosweep past the lower edge of its associated dividing wall so that theoutlet port in the deck passes sequentially underneath successivecompartments in the chamber.

Preferably, the decks are spaced so that the layers of pulp supportedthereby do not exceed a maximum value at which the pulp at the bottom ofa layer contains a predetermined minimum gaseous content as outlinedabove. The decks may be essentially horizontal but may be somewhatinclined towards the center.

In order to more effectively introduce oxygen at each transfer level andin order to minimise any tendency for compaction, the chamber betweenthe top two decks may be somewhat less in height than all subsequentchambers;

The chamber between the two top floors may thus be not more than 3.00metres and preferably not more than 1.20 metres high, although forapparatus handling pulps well fluffed and not below 20 percentconsistency this restriction need not apply.

Prefeably each stationary dividing wall extends over substantially theentire height of its chamber. Thus the spaces above each deck aredivided into a plurality of separate upright pulp holding apartments,each deck being arranged for pulp to be transferred sequentially fromdifferent compartments above it to the space below it. The pulp from acompartment above a deck may be transferred to a compartment below thedeck and preferably, the pulp is transferred in a uniform manner withoutany hang-ups.

The means for moving the decks comprises a shaft which extends axiallyin the pressure vessel, the decks being mounted on the shaft forrotation therewith.

Each stationary dividing wall has a lower edge so located relative toits associated movable deck to retain the pulp in each compartment as adiscrete batch substantially without intermixing with other batchesduring movement along the pathway.

The outlet ports of adjacent movable decks are angularly off-set fromone another with respect to a central axis of the vessel, so thatsubstantially all the pulp passing into a compartment is retained inthat compartment for a controlled residence period.

Means may also be provided for fluffing pulp on introducing it to saidpathway. Preferably a rotary feeding device is provided for distributionof pulp above the upper floor.

The pressure vessel may beprovided with a pulp inlet towards its upperend and with a pulp outlet towards its lower end. The vessel may furtherbe provided with one or more steam and oxygen gas inlets to maintain thetemperature and pressure at a selected value such as 130C and 140p.s.i.g.

The pressure vessel towards its outlet may be provided with means tore-unite the pulp into a single column which may be washed, diluted,cooled and discharged from the vessel in known manner.

An advantage of the apparatus is that the shaft may enter the vessel atthe bottom below the level of the diluted pulp in this way no glandsoperate in the gaseous phase thus simplifying the shaft sealing problem.

An added advantage is that the compartments are run substantially fullwhich is an important factor in equipment operating under pressure.

For a clear understanding of the invention a preferred embodiment willnow be described by way of example with reference to the accompanyingdrawings in which:

FIG. 1 is a vertical sectional view of one embodiment of a pulp treatingapparatus according to the invention, FIG. 2 is a cross-section on theline IIII in FIG. 1, FIG. 3 is a cross-section on the line IIIIII inFIG. 1; FIG. 4 is a cross-section on the line IV-IV in FIG.

FIG. 5 is a cross-section on the line V--V in FIG. 1; and

FIG. 6 is a cross-section on the line VIVI in FIG.

Referring to FIGS. 1 to 6, reactor 1 comprises a cylindrical pressurevessel which is provided with a plurality of vertically spaced pulpsupport decks 2a, 2b, 2c and 2d in the form of spaced discs, mounted onshaft 3 which is located co-axially within reaction vessel 1 and whichis adapted to be rotatably driven by a motor (not shown). The number ofdecks may be increased to any convenient number and are not limited to 4as in this example.

A plurality of vertically extending, radially disposed stationarydividing walls 5 are located above each of the decks 2a, 2b, 2c and 2dto divide the chamber above each deck into a plurality of uprightcompartments 6 disposed radially about shaft 3. The dividing walls 5 atthe different levels are vertically aligned to provide correspondingcompartments 6 at the different levels which are vertically aligned. Thedividing walls 5 above the deck 2b are about 1.20 metres high and thedividing walls 5 above the decks 2c and 2d are about 1.30 metres high.The height of the chamber above deck is more than l.20 metres in orderto be able to provide the pulp layer of 1.20 metres in the chamber abovedeck 2!).

As can be seen from FIGS. 2 to 6 of the drawings, each deck is providedwith an aperture 7 which may for instance conform to the cross-sectionalconfiguration of the compartments 6, the apertures of the various decksbeing angularly off-set preferably by an angle greater than a butusually less than 2a where a corresponds to the angle subtended by onecompartment 6 at the axis. The angle is so adjusted that all the pulppassing into the compartment is retained for the controlled residenceperiod.

At its upper end, reaction vessel 1 is provided with a pulp inlet 8 anda rotary feeding device 9 which is mounted on shaft 3 and which isadapted to direct incoming pulp into the various compartments 6 aboveupper deck 2a. Device 9d is provided to fluff incoming pulp. At itslower end, reaction vessel 1 is provided with dilution nozzles 14 andwith a scraper and extractor 10 for discharging treated pulp throughoutlet 11. The reaction vessel 1 is further provided at its upper endwith oxygen and steam inlets 12 and at its lower end with oxygen inlets13.

In operation, heated pulp pluschemicals and oxygen plus steam areintroduced into reaction vessel 1 at its upper end through inlets 8 and12 respectively. The oxygen is admixed with the pulp by the fluffingdevice and then directed into compartments 6 above upper deck 2a byfeeding device 9. The pulp may be premixed with oxygen before enteringreaction vessel 1, if necessary. By rotatably driving shaft 3, the decks2a, 2b, 2c and 2d are rotated so that the aperture7 in each deck passessuccessively underneath the compartments 6 above such deck. It will beappreciated that as each deck rotates, a batch of pulp equal in quantityto the layer supported by deck 2b drops out of each successivecompartment 6 above the deck into the corresponding compartment belowthe deck. Once the aperture 7 in a deck has passed a compartment 6 abovethe deck, the pulp in that compartment 6 above the deck remainssubstantially undisturbed until the aperture 7 reaches that compartmentagain during the next revolution of the deck. In this manner pulp istransferred compartment by compartment in stepwise fashion from theupper end of the pressure vessel 1 towards the bottom of the vessel fromwhere treated pulp is discharged through outlet 11. The pulp drops bygravity in a stepwise cascade so that the pulp is spread or loosened andoxygen in reaction vessel 1 has access and can penetrate evenly into thesmaller spaces and pores of the pulp before the pulp forms a horizontallayer again. It

will be appreciated that the pulp contained in reaction vessel 1 isdivided into a plurality of batches, on each of the decks 2a, 2b, 2c and2d by the dividing walls.

The lower region of each stationary dividing wall 5 is closely spacedfrom its movable floor or deck 2a, 2b, 2c and 2d so that the dividingwall 5 retains the pulp in each compartment 6 as a discrete batchsubstantially without intermixing with other batches during movementalong the pathway.

The speed of rotation of shaft 3 and therefore, of the decks, isdictated by the total height of the superimposed layers of pulp inreaction vessel 1 and the required reaction time. It will be appreciatedthat in the above example during each revolution less an angle equal tothe angle of off-set of the decks, the pulp moves down about 1.20metres, i.e. the height of the pulp layer above deck 2b. Where, forexample, the total height of pulp in the reaction vessel is 15 metres;and the required reaction time is 30 minutes, and the pulp is requiredto move downwardly at a speed of 0.5 metres per minute, the shaft speedwould be (0.5/L2) 0.41 r.p.m.

It will be appreciated that the reaction time can be changed by changingthe speed of rotation of the shaft and thus of the decks. In that event,it may also be necessary to change the rate of pulp introduction intoand extraction from the reaction vessel for satisfactory continuousoperation.

We claim:

1. Apparatus for treatment of cellulose pulp with oxygen comprising avertical pressure vessel; spaced decks located transversely within thepressure vessel and defining a series of pulp trapping chambers in thevessel; an outlet port from each chamber located in the deck thereof andcommunicating with an adjacent chamber so as to provide a path for pulpto move gravitationally from chamber to chamber, each outlet port havinga radial dimension substantially equal to the effective radial dimensionof the chamber; dividing walls projecting upwardly transverse to thedeck of each chamber to divide the chamber into a plurality ofcompartments; means for contacting the pulp with oxygen gas duringpassage along the pathway; said dividing walls being stationary; andmeans for moving the decks of the chambers relative to the dividingwalls to permit the deck of each chamber to sweep past the lower edge ofits associated dividing wall so that the outlet port in the deck passessequentially underneath successive compartments in the chamber.

2. Apparatus as claimed in claim 1 wherein the chamber between the toptwo movable decks is less in height than all subsequent chambers.

3. Apparatus as claimed in claim 2 wherein the chamher between the toptwo movable floors is not more than 3 meters high.

4. Apparatus as claimed in claim 1 wherein each stationary dividing wallextends substantially the entire height of its associated chamber.

5. Apparatus as claimed in claim 1 wherein the means for moving thedecks comprises a shaft which extends axially in the pressure vessel,the decks being mounted on the shaft for rotation therewith.

6. Apparatus as claimed in claim 1 wherein each stationary dividing wallhas a lower edge so located relative to its associated movable deck toretain the pulp in each compartment as a discrete batch substantiallywithout intermixing with other batches during movement along the path.

7. Apparatus as claimed in claim 1 wherein the outlet ports of adjacentmovable decks are angularly off-set from one another with respect to acentral axis of the vessel, so that substantially all the pulp passinginto a compartment is retained in that compartment for a controlledresidence period. I

8. Apparatus as claimed in claim 1 comprising means for fluffing pulp onintroducing the same to said path.

2. Apparatus as claimed in claim 1 wherein the chamber between the toptwo movable decks is less in height than all subsequent chambers. 3.Apparatus as claimed in claim 2 wherein the chamber between the top twomovable floors is not more than 3 meters high.
 4. Apparatus as claimedin claim 1 wherein each stationary dividing wall extends substantiallythe entire height of its associated chamber.
 5. Apparatus as claimed inclaim 1 wherein the means for moving the decks comprises a shaft whichextends axially in the pressure vessel, the decks being mounted on theshaft for rotation therewith.
 6. Apparatus as claimed in claim 1 whereineach stationary dividing wall has a lower edge so located relative toits associated movable deck to retain the pulp in each compartment as adiscrete batch substantially without intermixing with other batchesduring movement along the path.
 7. Apparatus as claimed in claim 1wherein the outlet ports of adjacent movable decks are angularly off-setfrom one another with respect to a central axis of the vessel, so thatsubstantially all the pulp passing into a compartment is retained inthat compartment for a controlled residence period.
 8. Apparatus asclaimed in claim 1 comprising means for fluffing pulp on introducing thesame to said path.